The desire to increase the efficiency of turbomachines, in particular in the field of aviation, and also to reduce fuel consumption and polluting emissions of gases and unburned fuel, have led to fuel combustion under conditions that are closer to stoichiometric. This situation is accompanied by an increase in the temperature of the gases leaving the combustion chamber and heading towards the turbine.
Consequently, it has been necessary to adapt the materials used in the turbine to this increase in temperature, by improving techniques for cooling turbine blades (hollow blades), and/or by improving the properties of such materials in terms of their ability to withstand high temperatures. This second technique, in combination with the use of superalloys based on nickel and/or cobalt, has led to various solutions including depositing a coating of thermally insulating material known as a thermal barrier.
Under steady operating conditions and with a part that is cooled, the ceramic coating makes it possible to establish a temperature gradient through the coating with a total amplitude that can exceed 200° C. for a coating that is about 150 micrometers (μm) thick. The operating temperature of the underlying metal forming the substrate for the coating is thus reduced by that gradient, thereby leading to significant savings in the volume of cooling air that is needed, and to significant improvements in the lifetime of the part and in the specific fuel consumption of the turbine engine.
Naturally, in order to improve the properties of the thermal barrier, and in particular its bonding with the substrate, it is possible to include an underlayer between the substrate and the coating. In particular, it is known to make an underlayer constituted by one or more aluminides, comprising in particular a nickel aluminide optionally including a metal selected from platinum, chromium, palladium, ruthenium, iridium, osmium, rhodium, or a mixture of these metals, and/or a reactive element selected from zirconium (Zr), hafnium (Hf), and yttrium (Y), and/or an alloy of the MCrAlY type, where M is a metal selected from nickel, cobalt, iron, or a mixture of those metals.
Usually, ceramic coatings are deposited on the part to be coated either by a spraying technique (in particular plasma spraying), or by a physical vapor deposition technique, i.e. by evaporation, in particular by electron beam physical vapor deposition (EB-PVD) forming a coating that is deposited in an enclosure for vacuum evaporation under electron bombardment).
With a sprayed coating, a zirconia-based oxide is deposited by plasma spraying type techniques, leading to the formation of a coating constituted by a stack of droplets that are molten and then quenched by shock, being flattened and stacked so as to form a deposit that is imperfectly densified and that has a thickness that generally lies in the range 50 μm to 1 millimeter (mm).
A physically deposited coating, and in particular a coating deposited by evaporation under electron bombardment, leads to a coating that is made up of an assembly of columns directed substantially perpendicularly to the surface to be coated, over a thickness lying in the range 20 μm to 600 μm. Advantageously, the space between the columns allows the coating to compensate effectively for the thermomechanical stresses that are due, at operating temperatures, to differential expansion relative to the superalloy substrate, and to centrifugal mechanical stresses due to rotation of the blades. Parts can thus be obtained having long lifetimes when subjected to thermal fatigue at high temperature.
Conventionally, such thermal barriers thus lead to a discontinuity in thermal conductivity between the outer coating of the mechanical part, comprising said thermal barrier, and the substrate of the coating forming the material that constitutes the part.
Usually, it is found that thermal barriers which lead to a large discontinuity in thermal conductivity suffer from a high risk of separation between the coating and the substrate, and more precisely at the interface between the underlayer and the ceramic thermal barrier.
At present, it is desired to obtain thermal barrier compositions which enable mechanical parts to withstand surface temperatures of about 1500° C., i.e. up to about 1300° C. within the substrate. The thermal barriers presently in use enable mechanical parts to withstand surface temperatures of about 1200° C. to 1300° C., i.e. about 1000° C. to 1100° C. within the substrate.
It is known to make use of a thermal barrier that is obtained from a base material constituted by zirconia possessing a coefficient of expansion that is close to that of the superalloy constituting the substrate, and of thermal conductivity that is quite low.
The present invention relates to the type of coating that is obtained by evaporating a target under an electron beam. The targets used are subjected to thermal shock when they are irradiated by the electron beam, which thermal shock can lead to the target breaking, in particular if the target presents defects and/or irregularities. When the target breaks, it is no longer usable in practice, since it is no longer capable of delivering material by evaporation in regular manner.
Patent application EP 1 055 743 relates to a material that can be deposited by electron beam evaporation, in which it is desired to compensate, at least in part, for the change in volume of the material due to the thermal expansion that occurs when the temperature rises due to the irradiation, by means of the 4% volume reduction that is induced by the phase transition between monoclinic zirconia which transforms into tetragonal zirconia as temperature rises from 500° C. to 1200° C. More precisely, action is taken on a broad distribution of particle sizes for the powder of monoclinic structure so as to ensure that this compensation takes place over a quite broad range of temperature values.
EP 1 055 743 also provides for the presence of monoclinic zirconia at a concentration of 25% to 90%, or preferably in the range 40% to 85%, for the purpose of improving ability to withstand thermal shock. As in DE 4 302 167, this improved resistance to thermal shock comes from the appearance of microcracks during the phase transition between the tetragonal phase and the monoclinic phase while temperature is falling, which microcracks are capable of absorbing the thermal shock energy so as to prevent cracks from propagating, and thus prevent the material from breaking. According to EP 1 055 743, the two above-mentioned roles of the monoclinic zirconia serve to increase resistance to thermal shock.
According to EP 1 055 743, the targets are unusable outside those ranges of values. More precisely, when the monoclinic phase content of the zirconia is less than 25%, thermal expansion during evaporation is compensated to a lesser extent by the volume reduction during phase transformation, and the proportion of microcracks is too small, thereby limiting resistance to thermal shock. When the monoclinic phase content of the zirconia is greater than 90%, the volume expansion induced by the phase change between tetragonal zirconia transforming into monoclinic zirconia during the cooling that follows the temperature rise inherent in evaporation is too great, thereby leading to cracks (seams or quenching cracks) greatly reducing the strength of the target and possibly leading to breakage thereof.